Thermohaline Mixing: Does it Really Govern the Atmospheric Chemical Composition of Low-mass Red Giants

Abstract

First results of our three-dimensional numerical simulations of thermohaline convection driven by 3He burning in a low-mass red giant branch (RGB) star at the bump luminosity are presented. They confirm our previous conclusion that this convection has a mixing rate that is a factor of 50 lower than the observationally constrained rate of RGB extra-mixing. It is also shown that the large-scale instabilities of the salt-fingering mean field (those of the Boussinesq and advection-diffusion equations averaged over length and timescales of many salt fingers), which have been observed to increase the rate of oceanic thermohaline mixing up to one order of magnitude, do not enhance the RGB thermohaline mixing. We speculate on possible alternative solutions of the problem of RGB extra-mixing, among which the most promising one that is related to thermohaline mixing takes advantage of the shifting of the salt-finger spectrum toward larger diameters by toroidal magnetic field.